Tactic 2: Conservation of Linear Momentum
For problems involving the conservation of linear momentum, first make sure that you have chosen a closed. isolated system. Closed means that no matter (no particles) passes through the system boundary in any direction. Isolated means that the net external acting on the system is zero.
Next. select two appropriate states of the system (which you . may choose to call the initial state and the final state) and write’ expressions for the linear momentum of the system in each of these two states. In writing these expressions, make sure that you know what inertial reference frame you are using. and make sure also that you include the entire system, not missing any part of it and not including objects that do not belong to your system.